The ongoing demand for faster (higher data rate) wireless communication products is currently the subject of a number of proposals before the IEEE 802.11 committee, that involve the use of a new standard for the 2.4 GHZ portion of the spectrum, which FCC Part 15.247 requires be implemented using spread spectrum techniques that enable intra-packet data rates to exceed 10 Mbps Ethernet speeds. The 802.11 standard presently covers only one and two Mpbs data rates, that use either frequency hopping (FH) or direct sequence (DS) spread spectrum (SS) techniques. The FCC requirement for the use of spread spectrum signaling takes advantage of inherent SS properties that make the signals more robust to inadvertence interference--by lowering the average transmit power spectral density, and through receiver techniques which exploit spectral redundancy and thereby combat self-interference created by multipath distortion.
As shown in FIG. 1, the power delay profile (PDF) 10 of a transmitted signal due to multipath distortion within an indoor WLAN system, such as the reduced complexity example illustrated in FIG. 2, exhibits a largely exponentially-decayed Rayleigh fading characteristic. Physical aspects of the indoor transmission environment driving this behavior are the relatively large number of reflectors (e.g., walls) within the building, such as shown at nodes 12 and 13, between a transmitter site 14 and a receiver site 15, and the propagation loss associated with the respectively later time-of-arrival propagation paths t.sub.1, t.sub.2 and t.sub.3, which contain logarithmically weaker energies.
The power delay profile of the signal is the variation in mean signal power with respect to its power dispersed across time. The mean power level of the signal establishes the variance of its corresponding Rayleigh components. A principal aspect of the exponentially decayed multipath effect is due to the fact that a signal's propagation delay t.sub.i is proportional to the total distance traveled, so that, on-average, the strongest (minimal obstruction containing) transmission paths are those whose signals are the earliest to arrive at the receiver. In a given stochastic occurrence, a first to arrive, direct or line-of-sight path from the transmitter site 14 to the receiver site 15 may encounter an attenuating medium (such as one or more building walls and the like), while a later arriving signal reflected off a highly reflective surface and encounter no attenuating media may be have a larger channel impulse response (CIR) than the first-to-arrive signal. However, on average, such occurrences are few in number relative to the number of echo signals which follow the CIR peak.
In terms of a practical application, the root mean squared (RMS) delay spread of a multipath channel may range from 20-50 nsec for small office and home office (SOHO) environments, 50-100 nsec for commercial environments, and 100-200 nsec for factory environments. For exponentially faded channels, the (exponential) decay constant is equal to the RMS delay spread. For relatively low signal bandwidths (less than 1 MHz), fading due to multipath is mostly `flat`. However, at bandwidths above 1 MHz, for example at the 10 MHz bandwidth required by a direct sequence spread spectrum (DSSS) system to attain the above-referenced higher data rate of 10 Mbps, fading becomes selective with frequency, constituting a serious impediment to reliable communications over a multipath channel. Thus, multipath distortion within a WLAN environment can cause severe propagation loss over the ISM band.
A preferred mechanism to counter this severe frequency-selective multipath distortion problem is a channel-matched correlation receiver, commonly referred to as a `RAKE` receiver. For successful RAKE receiver operation, it is necessary to use a DSSS structure having a transmitted bandwidth larger than the information bandwidth. In a DSSS signal structure, a respective codeword is formed of a sequence of PN code `chips`. The term `codeword`, rather than `symbol`, is employed here to avoid confusion between `chips` and codewords. The DSSS chips may be transmitted using a relatively simple modulation scheme such as QPSK, and codeword chips may be fixed as in a signature sequence, or they may be pseudo random.
In addition, phase modulation of the codeword may be used to convey information. Namely, to impart additional bits of information per codeword, the codeword may be shifted in phase. For example two additional bits may be used to provide quadrature (ninety degree) phase shift increments: 0.degree., 90.degree., 180.degree. and 270.degree.. The codeword's chips may be selected from a multi-codeword set, where M bits select a particular codeword out of N codewords that make up the multi-codeword set. An example of such a scheme is the use of Walsh or Hadamard codes for the codeword set.
For the above-referenced 2.4 GHz spectrum, the IEEE 802.11 standards committee has proposed using an eight bit encoding scheme, in which six bits select one of N=64 multi-chip codewords, and the remaining two bits define one of four possible (quadrature) phases of the selected codeword.
As diagrammatically illustrated in FIG. 3, in a channel-matched correlation or RAKE receiver, the received (spread) signal is coupled to a codeword correlator 31, the output of which (shown as a sequence of time-of-arrival impulses 32-1, 32-2, 32-3) is applied to a coherent multipath combiner 33. The codeword correlator 31 contains a plurality of correlators each of which is configured to detect a respectively different one of the codewords of the multi-codeword set. The coherent multipath combiner may be readily implemented as a channel matched filter (whose filter taps have been established by means of a training preamble prior to commencement of a data transmission session). The output of the coherent multipath combiner 33 is coupled to a peak or largest value detector 35, which selects the largest output produced by the coherent multipath combiner as the transmitted codeword. Since the RAKE receiver is a linear system, the order of the operations carried out by the channel matched filter (coherent multipath combiner) 33 and codeword correlator 31 may be reversed, as shown in FIG. 4, wherein the channel matched filter 33 is installed upstream of the codeword correlator 31.
A RAKE receiver works reasonably well, since it coherently combines the multipath received signals plus echoes into a single composite signal. By proper choice of the codewords that make up the codeword set, the echoes can be effectively eliminated during codeword correlation. Ideally, each codeword of the set has the following properties: 1--an impulse auto-correlation function; 2--it is mutually orthogonal (has a zero cross-correlation function) to all other codewords of the set; 3--it is long relative to the multipath spread; and 4--it has the same energy of each of the other N codewords of the set.
If properties 2 and 4 are absent, the RAKE receiver must establish an orthogonal basis and account for the imbalance, just as in quadrature amplitude modulation--a receiver complexity issue. Also, the codewords need not be long relative to the multipath spread, so long as the codewords are impulsive and have zero cross-correlation functions (namely, no intercodeword or intersymbol interference (ISI)). Interchip interference impacts only receiver energy (if the impulsive auto-correlation property is absent). Although an optimal RAKE receiver imparts orthogonality to the codeword correlator output and make a decision by observing all correlator outputs, no RAKE receiver is ideal, since it is effectively impossible to generate codewords having impulsive auto-correlation functions and zero cross-correlation functions.
In addition, for severe multipath, in order to minimize degradation due to intercodeword interference, the codeword length must be very large (e.g., on the order of 64, 128, 256 or above, as used in military applications). However, in commercial environments, the number of chips per codeword must be limited in order to maximize useable data bandwidth. Since the extent of codeword bleedover increases as the number of chips per codeword is reduced, where multipath distortion is significant, a very small codeword chip density may cause codeword energy bleedover/leakage across multiple codewords. The problem therefore is how to optimize the signal-to-noise ratio of the output of the RAKE receiver using such less than ideal codewords.